Perhaps three of the most notorious examples of actions by the victorious Allies which involved mass killings were:
The Katyn Forest massacre where over 22,000 Polish nationals mostly military officers and police officers but also civilians, were massacred by units of the NKVD (the Soviet secret police).
The aerial firebombing of Hamburg, Dresden and other German towns by the British. The official figure for the victims of the firebombing of Dresden was 35,000 but the city was full of refugees fleeing the advancing Red Army so the actual death toll may well have been much higher.
The dropping of atomic bombs on the Japanese cities of Hiroshima and Nagasaki by the United States in 1945. The official estimate of deaths from the explosions in the two cities is around 200,000 but this figure does not include those who died later from radiation. At the time the bombings were justified as essential to force the Japanese to surrender and thereby save the lives of Allied forces. However, both General Eisenhower, Supreme Allied Commander Europe and General MacArthur, Supreme Allied Commander Japan, opposed the use of atomic weapons at that time.